Method of expressing a G-protein coupled receptor protein

ABSTRACT

The present invention relates to fusion proteins for the expression of G-protein coupled receptor proteins (GPCR) with the fusion partners, as inserted fragments, from mammalian cells. The fusion partners are from a fragment of APJ protein (“the APJ protein fragment”) or a fragment with homology of more than 90% similarity to the APJ protein fragment; or a fragment of RGS16 protein (the “RGS16 protein fragment”) or a fragment with homology of more than 90% similarity to the RGS16 protein fragment; or the fragment of DNJ protein (the “DNJ protein fragment”) or a fragment with homology of more than 90% similarity to DNJ protein fragment. The fusion expression of GPCR with the above mentioned fusion partners can improve the protein yield and stability when purified from cells. Therefore, these fusion protein partners can be widely used for the study of GPCR proteins.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims the priority benefit of U.S. application Ser. No. 14/633,153, filed on Feb. 27, 2015, now allowed, which is a continuation-in-part application of PCT application Ser. No. PCT/CN2014/070537, filed on Jan. 13, 2014, which claims the priority benefit of China application no. 201310577289.9, filed on Nov. 18, 2013. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to genetic engineering and specifically fusion expression of G-protein coupled receptors (GPCRs).

2. Description of Related Art

G-protein coupled receptors (GPCRs) are a large family of transmembrane protein receptors. All GPRCs share a common structural feature, that is, an extracellular N-terminus, followed by seven transmembrane α-helices connected by three intracellular and three extracellular loops and finally an intracellular C-terminus.

The extracellular regions often have glycosylated residues. The C-terminus and the intracellular loop between the fifth and sixth transmembrane helical regions together form the G protein binding site. There are 800 known GPCRs so far, which can be classified into six classes: Class A (or 1) (Rhodopsin-like); Class B (or 2) (Secretin receptor family); Class C (or 3) (Metabotropic glutamate/pheromone); Class D (or 4) (Fungal mating pheromone receptors); Class E (or 5) (Cyclic AMP receptors); and Class F (or 6) (Frizzled/Smoothened). [Friedricksson et al., Mol. Pharmacol. 63 (6):1256-1272, 2003: and Friedricksson et al., Mol. Pharmacol. 67 (5):1414-1425, 2005]. There is little nucleotide sequence homology between the GPCRs classes.

By coupling with different G proteins, the various GPCRs react to a vast array of extracellular signals, leading to a series of physiology effects including neural transmission, smell, taste, vision and cellular metabolism, differentiation, reproduction and endocrine responses.

Numerous diseases are known to be associated with GPCRs. More than 40% of modern drugs, and over half of the thousands of drugs on the market target GPCRs. These GPCR-targeting drugs are effective treatments of pain, cognizance impairment, high blood pressure, ulcer, nasal inflammation and asthma. Due to the important physiological roles of GPCRs, their structure and function have been intensively studied.

However, wild type GPCRs are unstable in vitro and it is difficult to obtain pure and stable form. Recently, several research groups reported methods used to improve the stability of GPCRs, including (1) insertion of an E. coli T4 phage lysozyme T4L between the ICL3 (intracellular loop 3) and the N-terminus. This approach has been successfully applied to the studies of A2a receptor, CXCR4 receptor, beta-2 adrenergic receptor, D3 dopamine receptor, S1P1 receptor etc. The modification of GPCR with T4L has led to high expression and high yield, and eventually to a high-resolution crystal structure. [Rasmussen et al., Crystal structure of the human beta2 adrenergic G-protein-coupled receptor, Nature 450: 383-387, 2007; Wu et al., Structures of the CXCR4 chemokine GPCR with small-molecule and cyclic peptide antagonists Science 330: 1066-1071, 2010; Chien et al., Structure of the human dopamine D3 receptor in complex with a D2/D3 selective antagonist, Science 330: 1091-1095, 2010; Xu et al., Structure of an agonist-bound human A2A adenosine receptor, Science 332: 322-327, 2011; Hanson et al., Crystal structure of a lipid G protein-coupled receptor; Science 335: 851-855, 2012; and Zou et al., N-terminal T4 lysozyme fusion facilitates crystallization of a G protein coupled receptor Plos One 7: e46039-e46039 2012]. (2) Insertion of bacterial Bril protein in the N-terminus or ICL3. This has been successfully applied to GPCRs such as adenosine A2a receptor, Nociceptin/orphanin FQ receptor, 5HT1b, 5HT2b and SMO receptor, leading to successful determination of their crystal structures. [Liu, W. et al. Structural basis for allosteric regulation of GPCRs by sodium ions, Science 337: 232-236, 2012. Thompson, A. A. et al. Structure of the nociceptiniorphanin FQ receptor in complex with a peptide mimetic Nature 485: 395-399, 2012. Wang, C. et al. Structural Basis for Molecular Recognition at Serotonin Receptors Science 2013. Wang, C. Structure of the human smoothened 7TM receptor in complex with an antitumor agent Nature 2013]. (3) Mutation screening of GPCRs for mutants which possess improved stability with unaffected protein structure and function. This approach has been successfully demonstrated the stable preparation of A2a and beta-1 adrenergic receptor with high yield and high resolution crystal structures [Lebon, G. et al. Agonist-bound adenosine A2A receptor structures reveal common features of GPCR activation. Nature 474: 521, 2011. Warne, A. et al. Structure of a beta1-adrenergic G-protein-coupled receptor Nature 454: 486, 2008]. (4) Using antibody to stabilize the configuration of GPCRs. Using this approach, the Brian Kobilka Laboratory of Stanford University obtained a high resolution crystal structure of beta-2 adrenergic receptor [Bokoch M. P. et al., Ligand-specific regulation of the extracellular surface of a G-protein-coupled receptor, Nature 463: 108-112, 2010].

So far, all proteins used as fusion partners to stabilize GPCRs are prokaryotic proteins, and there has been no report of using a eukaryotic fusion protein partner. Use of eukaryotic fusion protein partners for GPCR protein expression may be advantageous since all GPCRs are present in eukaryotic cells. Therefore it would be desirable to find eukaryotic protein partners for GPCRs fusion expression. Furthermore, even though T4L and Bril proteins have successfully been applied to some GPCRs for expression and purification, they are not useful for many other GPCRs. Thus, additional fusion protein partners are highly desirable.

SUMMARY OF THE INVENTION

In some embodiments, the present invention provides a number of novel fusion protein partners as inserted fragments which can be used for expression of GPCRs so as to provide more options for GPCR expression.

Specifically, this invention provides eukaryotic fusion proteins for GPCR expression, i.e. APC, RGS16 and DNJ protein fragments, and demonstrates the successful use of these protein fragments in fusion expression of GPCR proteins.

The fusion protein partners for GPCR expression according to the present invention are characterized by: being from a eukaryotic source, and selected from a fragment of APC protein (the “APC protein fragment”) or a polypeptide having greater than 90% amino acid sequence identity to the APC protein fragment; a fragment of RCS16 protein (the “RCS16 protein fragment”) or a polypeptide having greater than 90% amino acid sequence identity to the RCS16 protein fragment; or a fragment of DNJ protein (the “DNJ protein fragment”) or a polypeptide having greater than 90% amino acid sequence identity to the DNJ protein fragment.

The APC protein fragment has an amino acid sequence as shown in SEQ ID NO:1, which is encoded by a DNA fragment having a nucleic acid sequence as shown in SEQ ID NO: 4; The RCS16 protein fragment has an amino acid sequence as shown in SEQ ID NO:2, which is encoded by a DNA fragment having a nucleic acid sequence as shown in SEQ ID NO:5.

The DNJ protein fragment has an amino acid sequence as shown in SEQ ID NO:3, which is encoded by a DNA fragment having a nucleic acid sequence as shown in SEQ ID NO: 6.

This invention involves the engineering of GPCR at the N-terminus, C-terminus or ICL3 region through the insertion of the APC, RGS16, or DNJ protein fragment to stabilize GPCR proteins.

In one embodiment, these three protein fragments are used for fusion expression of the A2a protein. When using the APC fragment, the resultant fusion protein comprises an amino acid sequence shown in SEQ ID NO: 7, which is encoded by a DNA fragment having a nucleic acid sequence as shown in SEQ ID NO: 10; when using the RGS16 fragment, the resultant fusion protein comprises an amino acid sequence shown in SEQ ID NO: 8, which is encoded by a DNA fragment having a nucleic acid sequence as shown in SEQ ID NO: 11; and when using the DNJ fragment, the resultant fusion protein comprises an amino acid sequence shown in SEQ ID NO: 9, which is encoded by a DNA fragment having a nucleic acid sequence as shown in SEQ ID NO: 12. Amino acid sequences as shown in SEQ ID NOs: 7, 8 and 9 can be used for fusion expression of the A2a protein; wherein polynucleotides having a nucleotide sequence as shown in SEQ ID NO: 10, 11, or 12 can be introduced into plasmids, such as pFastBac1 (available from Life Technologies), PcDNA3.1 and PET21b for fusion expression of the A2a protein.

It is noted that a person ordinarily skilled in the art can construct a GPCR fusion expression vector comprising the APC, RGS16 or DNJ protein fragment, by using methods well known in the art, such as in vitro recombinant DNA techniques, DNA synthesis techniques, in vivo recombinant techniques. The GPCR fusion expression vector should for example comprise an appropriate promoter which controls mRNA synthesis.

Furthermore, the above constructed vector can be used to transfect or transform appropriate host cells by known methods in the art for culturing and harvest of the expressed protein. For example, using the Bac-to-Bac technique, SF9 cells can be transfected with the fusion expression vector comprising the PFastBac plasmid (see below).

The constructed fusion proteins of the present invention can be expressed in insect cells such as SF9, SF21 and Hive5 and also can be expressed in yeast and mammalian host cells such as 293 or CHO, to produce proteins with a wide variety of applications.

This invention provides mammalian fusion partners, as inserted fragments, for fusion expression of the GPCR: APC, RGS16 and DNJ and further provides the amino acid and DNA sequences of the fusion proteins.

Furthermore, this invention demonstrated application of the above mentioned novel fusion protein partners in the expression of the GPCR, i.e. the A2a receptor. The above mentioned. APC, RGS16 and DNJ protein fragments are inserted into the various regions of A2a (N-terminus, C-terminus or ICL3) and the related amino acid and gene sequences are provided. As a result, the expression yield and in vitro stability of the A2a receptor are greatly improved. These novel fusion proteins can be widely used in the studies of GPCR. The constructs of fusion expression of A2a receptor are provided along with the method of expression in baculo SF9 cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a : SDS gel analysis of A2a-APC fusion protein;

FIG. 1b : SDS gel analysis of A2a-RGS16 fusion protein;

FIG. 1c : SDS gel analysis of A2a-DNJ fusion protein;

FIG. 2a : Thermal stability measurement of A2a-APC fusion protein in the presence of substrate Adenosine;

FIG. 2b : Thermal stability measurement of A2a-RGS 16 fusion protein in the presence of substrate Adenosine;

FIG. 2c : Thermal stability measurement of A2a-DNJ fusion protein in the presence of adenosine;

FIG. 3a , Ultra performance liquid chromatography (UPLC) analysis of A2a-APC fusion protein;

FIG. 3b , UPLC analysis of A2a-RGS16 fusion protein;

FIG. 3c , UPLC analysis of A2a-DNJ fusion protein.

DESCRIPTION OF THE EMBODIMENTS Example 1: Preparation of Genes Coding for the Fusion Proteins

(1) (a) The APC protein fragment comprises an amino acid sequence of SEQ ID NO: 1; and its coding DNA sequence is shown in SEQ ID NO: 4.

(SEQ ID NO: 4) 5′-TCCACCGGCTACCTGGAGGAGCTGGAGAAGGAGCGCTCCCTGCTGCT GGCCGACCTGGACAAGGAGGAGAAGGAGAAGGACTGGTACTACGCCCAGC TGCAGAACCTGACCAAGCGCATCGACTCCCTGCCCCTGACCGAGAACTTC TCCCTGCAGACCGACATGACCCGCCGCCAGCTGGAGTACGAGGCCCGCCA GATCCGCGTGGCCATGGAGGAGCAGCTGGGCACCTGCCAGGACATGGAGA AGCGCGCCCAGCGCCGCATCGCCCGCATCCAGCAGATCGAGAAGGACATC CTGCGCATCCGCCAG-3′ Forward primer: (SEQ ID NO: 13) 5′-TTCCTGGCGGCGCGACGACAGCTGTCCACCGGCTACCTGGAGG-3′ Reverse primer: (SEQ ID NO: 14) 5′-CAGTGTGGACCGTGCCCGCTCCTGGCGGATGCGCAGGATGT-3′

The APC coding sequence was obtained by PCR.

(b) A2a DNA template was chemically synthesized based on human codon usage preference:

(SEQ ID NO: 19) 5′-ATGAAAACCATTATTGCGCTGAGCTATATTTTTTGCCTGGTGTTTGC GGATTATAAAGATGATGATGATGGCGCGCCGCCCATCATGGGCTCCTCGG TGTACATCACGGTGGAGCTGGCCATTGCTGTGCTGGCCATCCTGGGCAAT GTGCTGGTGTGCTGGGCCGTGTGGCTCAACAGCAACCTGCAGAACGTCAC CAACTACTTTGTGGTGTCACTGGCGGCGGCCGACATCGCAGTGGGTGTGC TCGCCATCCCCTTTGCCATCACCATCAGCACCGGGTTCTGCGCTGCCTGC CACGGCTGCCTCTTCATTGCCTGCTTCGTCCTGGTCCTCACGCAGAGCTC CATCTTCAGTCTCCTGGCCATCGCCATTGACCGCTACATTGCCATCCGCA TCCCGCTCCGGTACAATGGCTTGGTGACCGGCACGAGGGCTAAGGGCATC ATTGCCATCTGCTGGGTGCTGTCGTTTGCCATCGGCCTGACTCCCATGCT AGGTTGGAACAACTGCGGTCAGCCAAAGGAGGGCAAGAACCACTCCCAGG GCTGCGGGGAGGGCCAAGTGGCCTGTCTCTTTGAGGATGTGGTCCCCATG AACTACATGGTGTACTTCAACTTCTTTGCCTGTGTGCTGGTGCCCCTGCT GCTCATGCTGGGTGTCTATTTGCGGATCTTCCTGGCGGCGCGACGACAGC TGGCTGATCTGGAAGACAATTGGGAAACTCTGAACGACAATCTCAAGGTG ATCGAGAAGGCTGACAATGCTGCACAAGTCAAAGACGCTCTGACCAAGAT GAGGGCAGCAGCCCTGGACGCTCAGAAGGCCACTCCACCTAAGCTCGAGG ACAAGAGCCCAGATAGCCCTGAAATGAAAGACTTTCGGCATGGATTCGAC ATTCTGGTGGGACAGATTGATGATGCACTCAAGCTGGCCAATGAAGGGAA AGTCAAGGAAGCACAAGCAGCCGCTGAGCAGCTGAAGACCACCCGGAATG CATACATTCAGAAGTACCTGGAACGTGCACGGTCCACACTGCAGAAGGAG GTCCATGCTGCCAAGTCACTGGCCATCATTGTGGGGCTCTTTGCCCTCTG CTGGCTGCCCCTACACATCATCAACTGCTTCACTTTCTTCTGCCCCGACT GCAGCCACGCCCCTCTCTGGCTCATGTACCTGGCCATCGTCCTCTCCCAC ACCAATTCGGTTGTGAATCCCTTCATCTACGCCTACCGTATCCGCGAGTT CCGCCAGACCTTCCGCAAGATCATTCGCAGCCACGTCCTGAGGCAGCAAG AACCTTTCAAGGCACATCATCATCACCATCACCACCATCACCATTAA-3′ Forward primer (SEQ ID NO: 20) 5′-TATATTTTTTGCCTGGTGTTTGCGGATTATAAAGATGATGATGATGC GCCCATCATGGGCTCCTCGGT-3′ Reverse primer (SEQ ID NO: 21) 5′-CCTCCAGGTAGCCGGTGGACAGCTGTCGTCGCGCCG-3′

Using the above template and primers, the coding sequence for A2a (2-208) was obtained by PCR.

Forward primer:  (SEQ ID NO: 22) 5′-GAGCGGGCACGGTCCACACT-3′ Reverse primer:  (SEQ ID NO: 23) 5′-TTAATGGTGATGGTGGTGATGGTGATGATGATGTGCCTT-3′

Using the template and the primers above, the coding sequence for A2a (SEQ ID NO: 26, position 219-316) was obtained by PCR.

The coding sequence for A2a (SEQ ID NO: 26, position 2-208)-APC (SEQ ID NO: 1, position 1-104)-A2a (SEQ ID NO: 26, position 219-316) was prepared based on the protocol above. The restriction enzyme site EcoRI was introduced to the forward primer, while the XhoI site and the stop codon were introduced to the reverse primer.

(2) (a) The RGS16 protein fragment comprises an amino acid sequence of SEQ ID NO:2; and its DNA coding sequence is shown as SEQ ID NO:5.

(SEQ ID NO: 5) 5′-AACTTCTCCGAGGACGTGCTGGGCTGGCGCGAGTCCTTCGACCTGCT GCTGTCCTCCAAGAACGGCGTGGCCGCCTTCCACGCCTTCCTGAAGACCG AGTTCTCCGAGGAGAACCTGGAGTTCTGGCTGGCCTGCGAGGAGTTCAAG AAGATCCGCTCCGCCACCAAGCTGGCCTCCCGCGCCCACCAGATCTTCGA GGAGTTCATCTGCTCCGAGGCCCCCAAGGAGGTGAACATCGACCACGAGA CCCACGAGCTGACCCGCATGAACCTGCAGACCGCCACCGCCACCTGCTTC GACGCCGCCCAGGGCAAGACCCGCACCCTGATGGAGAAGGACTCCTACCC CCGCTTCCTGAAGTCCCCCGCCTACCGCGACCTGGCCGCCCAGGCCTCCG CCGCCTCC-3′ Forward primer: (SEQ ID NO: 15) 5′-GAGAACCTGTACTTCCAATCCAACTTCTCCGAGGACGTGCT-3′ Reverse primer: (SEQ ID NO: 16) 5′-ACACCGAGGAGCCCATGATGGGGGAGGCGGCGGAGGCCTG-3′

The RGS16 coding gene was obtained by PCR.

(b) Ata DNA template was chemically synthesized based on human codon usage preference:

(SEQ ID NO: 19) 5′-ATGAAAACCATTATTGCGCTGAGCTATATTTTTTGCCTGGTGTTTGC GGATTATAAAGATGATGATGATGGCGCGCCGCCCATCATGGGCTCCTCGG TGTACATCACGGTGGAGCTGGCCATTGCTGTGCTGGCCATCCTGGGCAAT GTGCTGGTGTGCTGGGCCGTGTGGCTCAACAGCAACCTGCAGAACGTCAC CAACTACTTTGTGGTGTCACTGGCGGCGGCCGACATCGCAGTGGGTGTGC TCGCCATCCCCTTTGCCATCACCATCAGCACCGGGTTCTGCGCTGCCTGC CACGGCTGCCTCTTCATTGCCTGCTTCGTCCTGGTCCTCACGCAGAGCTC CATCTTCAGTCTCCTGGCCATCGCCATTGACCGCTACATTGCCATCCGCA TCCCGCTCCGGTACAATGGCTTGGTGACCGGCACGAGGGCTAAGGGCATC ATTGCCATCTGCTGGGTGCTGTCGTTTGCCATCGGCCTGACTCCCATGCT AGGTTGGAACAACTGCGGTCAGCCAAAGGAGGGCAAGAACCACTCCCAGG GCTGCGGGGAGGGCCAAGTGGCCTGTCTCTTTGAGGATGTGGTCCCCATG AACTACATGGTGTACTTCAACTTCTTTGCCTGTGTGCTGGTGCCCCTGCT GCTCATGCTGGGTGTCTATTTGCGGATCTTCCTGGCGGCGCGACGACAGC TGGCTGATCTGGAAGACAATTGGGAAACTCTGAACGACAATCTCAAGGTG ATCGAGAAGGCTGACAATGCTGCACAAGTCAAAGACGCTCTGACCAAGAT GAGGGCAGCAGCCCTGGACGCTCAGAAGGCCACTCCACCTAAGCTCGAGG ACAAGAGCCCAGATAGCCCTGAAATGAAAGACTTTCGGCATGGATTCGAC ATTCTGGTGGGACAGATTGATGATGCACTCAAGCTGGCCAATGAAGGGAA AGTCAAGGAAGCACAAGCAGCCGCTGAGCAGCTGAAGACCACCCGGAATG CATACATTCAGAAGTACCTGGAACGTGCACGGTCCACACTGCAGAAGGAG GTCCATGCTGCCAAGTCACTGGCCATCATTGTGGGGCTCTTTGCCCTCTG CTGGCTGCCCCTACACATCATCAACTGCTTCACTTTCTTCTGCCCCGACT GCAGCCACGCCCCTCTCTGGCTCATGTACCTGGCCATCGTCCTCTCCCAC ACCAATTCGGTTGTGAATCCCTTCATCTACGCCTACCGTATCCGCGAGTT CCGCCAGACCTTCCGCAAGATCATTCGCAGCCACGTCCTGAGGCAGCAAG AACCTTTCAAGGCACATCATCATCACCATCACCACCATCACCATTAA-3′ Forward primer (SEQ ID NO: 24) 5′-CCCATCATGGGCTCCTCGGT-3′ Reverse primer (SEQ ID NO: 25) 5′-TTGGTACCGCATGCCTCGAGTTAATGGTGATGGTGGTGATGGTGATG ATGATGTGCCTT-3′

Using the template and the primers above, the coding sequence for Ata (219-316) was obtained by PCR.

The coding sequence for RGS16 (SEQ ID NO: 2, position 2-135)-A2a (SEQ ID NO: 26, position 2-316) was prepared based on the protocol above. The restriction enzyme site EcoRI was introduced to the forward primer, while the XhoI site and the stop codon were introduced to the reverse primer.

(3) (a) The DNJ protein fragment comprises an amino acid sequence of SEQ ID NO: 3; and its coding DNA sequence is shown in SEQ ID NO: 6.

(SEQ ID NO: 6) 5′-GGCTACTACGACGTGCTGGGCGTGAAGCCCGACGCCTCCGACAACGA GCTGAAGAAGGCCTACCGCAAGATGGCCCTGAAGTTCCACCCCGACAAGA ACCCCGACGGCGCCGAGCAGTTCAAGCAGATCTCCCAGGCCTACGAGGTG CTGTCCGACGAGAAGAAGCGCCAGATCTACGACCAGGGCGGC-3′ Forward primer: (SEQ ID NO: 17) 5′-GAGAACCTGTACTTCCAATCCGGCTACTACGACGTGCTGG-3′ Reverse primer: (SEQ ID NO: 18) 5′-ACACCGAGGAGCCCATGATGGGGCCGCCCTGGTCGTAGATC-3′

The DNJ coding sequence was obtained by PCR.

(b) A2a DNA template was chemically synthesized based on humancodon usage preference:

(SEQ ID NO: 19) 5′-ATGAAAACCATTATTGCGCTGAGCTATATTTTTTGCCTGGTGTTTGC GGATTATAAAGATGATGATGATGGCGCGCCGCCCATCATGGGCTCCTCGG TGTACATCACGGTGGAGCTGGCCATTGCTGTGCTGGCCATCCTGGGCAAT GTGCTGGTGTGCTGGGCCGTGTGGCTCAACAGCAACCTGCAGAACGTCAC CAACTACTTTGTGGTGTCACTGGCGGCGGCCGACATCGCAGTGGGTGTGC TCGCCATCCCCTTTGCCATCACCATCAGCACCGGGTTCTGCGCTGCCTGC CACGGCTGCCTCTTCATTGCCTGCTTCGTCCTGGTCCTCACGCAGAGCTC CATCTTCAGTCTCCTGGCCATCGCCATTGACCGCTACATTGCCATCCGCA TCCCGCTCCGGTACAATGGCTTGGTGACCGGCACGAGGGCTAAGGGCATC ATTGCCATCTGCTGGGTGCTGTCGTTTGCCATCGGCCTGACTCCCATGCT AGGTTGGAACAACTGCGGTCAGCCAAAGGAGGGCAAGAACCACTCCCAGG GCTGCGGGGAGGGCCAAGTGGCCTGTCTCTTTGAGGATGTGGTCCCCATG AACTACATGGTGTACTTCAACTTCTTTGCCTGTGTGCTGGTGCCCCTGCT GCTCATGCTGGGTGTCTATTTGCGGATCTTCCTGGCGGCGCGACGACAGC TGGCTGATCTGGAAGACAATTGGGAAACTCTGAACGACAATCTCAAGGTG ATCGAGAAGGCTGACAATGCTGCACAAGTCAAAGACGCTCTGACCAAGAT GAGGGCAGCAGCCCTGGACGCTCAGAAGGCCACTCCACCTAAGCTCGAGG ACAAGAGCCCAGATAGCCCTGAAATGAAAGACTTTCGGCATGGATTCGAC ATTCTGGTGGGACAGATTGATGATGCACTCAAGCTGGCCAATGAAGGGAA AGTCAAGGAAGCACAAGCAGCCGCTGAGCAGCTGAAGACCACCCGGAATG CATACATTCAGAAGTACCTGGAACGTGCACGGTCCACACTGCAGAAGGAG GTCCATGCTGCCAAGTCACTGGCCATCATTGTGGGGCTCTTTGCCCTCTG CTGGCTGCCCCTACACATCATCAACTGCTTCACTTTCTTCTGCCCCGACT GCAGCCACGCCCCTCTCTGGCTCATGTACCTGGCCATCGTCCTCTCCCAC ACCAATTCGGTTGTGAATCCCTTCATCTACGCCTACCGTATCCGCGAGTT CCGCCAGACCTTCCGCAAGATCATTCGCAGCCACGTCCTGAGGCAGCAAG AACCTTTCAAGGCACATCATCATCACCATCACCACCATCACCATTAA-3′ Forward primer (SEQ ID NO: 24) 5′-CCCATCATGGGCTCCTCGGT-3′ Reverse primer (SEQ ID NO: 25) 5′-TTGGTACCGCATGCCTCGAGTTAATGGTGATGGTGGTGATGGTGATG ATGATGTGCCTT-3′

Using the template and the primers above, the coding sequence for A2a (219-316) was obtained by PCR.

The coding sequence for DNJ (SEQ ID NO: 3, position 1-63)-A2a (SEQ ID NO: 26, position 2-316) was prepared based on the protocol above. The restriction enzyme site EcoRI was introduced to the forward primer, while the XhoI site and the stop codon were introduced to the reverse primer.

PCR was conducted under the following conditions: 0.2 μM each of PCR primer was added into a 50 μl reaction system containing PCR buffer, 1.5 mM MgSO₄, 200 μM dNTPs. After mixing thoroughly, a PCR cycle was run on a PCR cycler: denaturation at 94° C. for 5 minutes, denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extending at 68° C. for 2 minutes. This cycle was repeated for 30 times. Finally, the temperature was kept at 68° C. for 10 minutes. PCR product was validated by 1.2% agarose gel electrophoresis, and purified for the purpose of subcloning.

Example 2: Construction for the Fusion Protein Plasmid

PCR product from Example 1 and vector pFastBac1 (available from Life Technologies, with brand Invitrogen) were digested by restriction enzymes EcoRI and XhoI, and ligated together. The ligation product was then transformed into competent cells DH5a. The ligation product volume was not higher than 10% of the competent cells volume. The mixture was gently mixed, then incubated in ice for 30 minutes, followed by incubation at 42° C. and heat shock for 60 seconds, and transferred quickly onto ice for 120 seconds to cool down the cells. 400 μl LB medium was then added to the cells. After gently mixed, the cells were recovered and the plasmid-encoded antibiotic marker gene was expressed in a 37° C. shaker for 60 minutes at low speed. Then the cells was spun down by centrifuge at low speed for 2 minutes, kept with about 100 μl LB medium in the tube and the extra supernatant removed. The cells was then re-suspended in 100 μl medium, and plated on a 1.5% agar plate containing 100 μg/ml Ampicilin. The plate was then incubated at 37° C. overnight for 12-16 hours and colonies appeared. The clones were validated by DNA sequencing.

Example 3: Expression of the Fusion Proteins

2 μl (>100 ng/μl) of recombinant pFastBac plasmid from Example 2 was added into 100 μl DH10Bac E. coli competent cells. The recombinant pFastBac plasmid volume was not higher than 5% of the competent cells volume. The mixture was gently mixed, then incubated in ice for 30 minutes, followed by incubation at 42° C. and heat shock for 90 seconds, and transferred quickly onto ice for 120 seconds to cool down the cells. 800 μl LB medium was then added to the cells. After gently mixed, the cells was recovered and the plasmid-encoded antibiotic marker gene was expressed was expressed in a 37° C. shaker for 4 hours at low speed (at 250 rpm). 30 μl cells was plated on LB agar plates containing 50 μl/ml Kanamycin, 7 μg/ml Gentmicin, 10 μg/ml tetracycline, 100 μg/ml X-gal and 40 μg/ml IPTG. The plate was then incubated at 37° C. for 30-48 hours and blue and white colonies appeared. The white colonies were selected from the plates and inoculated into 5 ml fresh LB medium containing 50 μl/ml Kanamycin, 7 μg/ml Gentamicin, and 10 μg/ml tetracycline at 37° C. overnight. The cells were validated by PCR analysis. The PCR analysis indicated that the positive cells contained rBacmids. The rBacmids were transfected into SF9 cells with transfection reagent at 27° C. and incubated for 4-5 days and then the culture medium was collected as P1 virus. The sf9 cells were infected with lower MOI (0.01-0.1) with the P1 virus for 72 hours to get P2 virus, used for expression of target peptides. The SF9 cells with density of 2.0×10^6/ml were infected with the P2 virus with infection ratio of 1:100 (Volume/Volume) for 72 hours. The cells were harvested by 4° C. centrifugation at 1500 rpm for 5 minutes, spun down by centrifuge and then washed once with 0.01M PBS buffer.

Example 4: Purification of the Fusion Proteins

Insect cell pastes from 1 L cell culture were disrupted by thawing frozen cell pellets in 200 ml hypotonic buffer containing 10 mM HEPES (pH 7.5), 10 mM MgCl₂, 20 mM KCl and protease inhibitor cocktail, and then homogenized in ice by a homogenizer. After the homogenization, extensive washing of the isolated raw membranes was performed by repeated centrifugation in the same hypotonic buffer for 3 times, and then in a high osmotic buffer containing 1.0 M NaCl, 10 mM HEPES (pH 7.5), 10 mM MgCl₂, 20 mM KCl for 3 times, followed by Dounce homogenization to re-suspend the membranes in 10 mM HEPES (pH 7.5), 10 mM MgCl₂, 20 mM KCl, and 40% glycerol, and then flash-frozened with liquid nitrogen and stored at −80° C. until further use.

Purified membranes were thawed in ice in the presence of 4 mM theophylline, 2 mg/mL iodoacetamide. Membranes from 1 L cell culture were disrupted in 100 ml buffer, after incubation for 30 min at 4° C. The membranes were solubilized by incubation in the presence of 0.5% (w/v) DDM and 0.1% (w/v) cholesteryl hemisuccinate for 3 hours at 4° C. The unsolubilized material was removed by centrifugation at 160,000 g for 40 min.

The supernatant was incubated with 1 ml pre-equilibrated TALON MAC resin. After overnight binding, the resin was transferred into a gravity column, washed in turn with ten column volumes of 25 mM HEPES (pH 7.5), 800 mM NaCl, 10% (v/v) glycerol, 0.05% (w/v) DDM, 0.001% (w/v) CHS, 25 mM Imid, 10 mM MgCl₂, 8 mM ATP, followed by four column volumes of 50 mM HEPES (pH 7.5), 800 mM NaCl, 10% (v/v) glycerol, 50 mM imidazole, 0.05% (w/v) DDM, 0.01% (w/v) CHS. The receptor was eluted with 25 mM HEPES (pH 7.5), 800 mM NaCl, 10% (v/v) glycerol, 220 mM imidazole, 0.025% (w/v) DDM, 0.005% (w/v) CHS. Purified receptor was saved at −80° C. FIG. 1 shows SDS gel analysis of three fusion proteins.

All three fusion GPCR proteins have a high yield more than 1 mg/L.

Example 5: Thermal Stability of the Fusion Proteins

Prior to use, the dye stock was diluted 1:40 in dye dilution buffer and incubated for 5 mm at room temperature. 130 μl protein solution was pipetted, 0.13 μl diluted dye was added and mixed together. After 5 min incubation at room temperature, the reaction mixture was transferred to a sub-micro quartz fluorometer cuvette and heated in a controlled way with a ramp rate of 2° C./min in a Cary Eclipse spectrofluorimeter. The excitation wavelength was at 387 nm and the grating gap was 2.5 nm, while the emission wavelength was 463 nm and the grating gap was 5 nm. Assays were performed at a temperature ranging from 20° C. to 90° C. and the temperature was increased by 1° C. every minute. All data were processed with GraphPad Prism program and thermal stability value (Tm value) was calculated.

Following above procedure, the Tm value of all three fusion proteins were measured, as showed in FIG. 2.

The Tm value of A2a-APC, A2a-RGS16, A2a-DNJ were 50.8° C., 55.6° C. and 52° C. respectively. All Tm values were higher than 50° C., demonstrating that these fusion proteins have good thermal stability.

Example 6: Test of Fusion Protein Homogeneity

Detection was performed by Acquity H-Class Bio UPLC system from Waters, with Sepas SEC 250 column. The column was washed to the base line with an equilibrium buffer solution (25 mM HEPES, 500 mM NaCl, 2% glycerol, 0.05 DDM, 0.01% CHS, pH 7.5) before loaded, until no considerable variance. The sample was then added into the special 96-well plate and treated with integration using by the software of the instrument.

The three fusion protein samples were detected according to the above method, the results of which are shown in FIG. 3. The three fusion proteins have good homogeneity with single peak which is the major part for the protein samples. 

What is claimed is:
 1. A method of expressing a stabilized G-Protein Coupled Receptor (GPCR) protein, comprising: constructing an expression vector, wherein the expression vector comprises: a first polynucleic acid fragment that encodes the GPCR protein, wherein the GPCR protein is an A2a protein having an amino acid sequence of SEQ ID NO: 26; and a second polynucleic acid fragment that encodes an inserted fragment, wherein the inserted fragment comprises an APC protein fragment having an amino acid sequence of SEQ ID NO: 1, a RGS16 protein fragment having an amino acid sequence of SEQ ID NO: 2, or a DNJ protein fragment having an amino acid sequence of SEQ ID NO: 3; and transforming a host cell with the expression vector; and culturing the host cell to allow expression of the expression vector.
 2. The method according to claim 1, wherein the host cell is an E. Coli cell and the expression vector is pFastBac plasmid. 